In the prior art there were many ways of making a document safe from alteration. Prior art techniques, for the most part having been based upon the utilization of chemical formulations which are or may be employed as either an overlay on the top surface of the document to be protected or as an overprinted area on such documents. In some instances a chemical wash of the entire paper stock is utilized.
With the advent of xerographic color copiers such as the Xerox L-6500 color copier or similar machines and their proliferation, the problem of nefarious reproductions has dramatically increased. The quality of the color reproduction at this point in the art is such that it is very difficult, often impossible, to discern whether the copied document is the original or a color copy. The reproduction of checks, stock certificates, automobile title instruments, etc. can be readily accomplished. Criminals having access to them and to a color copier may effectively duplicate these negotiable instruments via copying the registrations and title instruments so that they can be matched with the stolen vehicle. Copying checks and other personal identification documents can also be rewarding to the criminal. As the copier systems will proliferate so will their usage and the opportunity for many persons to make improper copies for questionable use.
In copending application Ser. No. 766,590 filed Feb. 8, 1977, now abandoned entitled "Protection System for Documents", incorporated herein fully by reference, there has been disclosed a system for protection of documents which employs a masked warning mark which when copied appears on the copy due to its effective color density being above the color reproductive threshold density of the copier. The mask is of a color density which is below the color reproductive density of the copier. An overlay of the mask and the warning phrase which has a color density exceeding the color reproductive threshold density causes the warning to appear on color copies.
That system was developed as an improvement over U.S. Pat. No. 3,802,724. It was developed as a solution to the color copier problems even in view of the fact that, as was known among those working on this problem in private laboratories, the resolving power of the Xerox L-6500 color copier causes larger dots to stand out prominently while smaller dots become less pronounced over a wide range of color copier settings. Experimentation had been done utilizing two different common tone screens as for instance 65 lines per inch and 133 lines per inch of substantially equal density, but the result was an effect which was not satisfactory. Accordingly the density threshold approach was conceived and implemented as described in the aforementioned application.
Modification of the exposure and/or development times of the master negatives could produce a sufficiently uniform tone and conceal the cancellation phrase from the casual observer. However, careful observation by a more critical observer permitted one to distinguish the cancellation phrase which was hidden by the dual screens of substantially equal density.
Accordingly the system in which a mask is of a color density which is below the color reproductive density of the copier and the overlay of the mask and warning phrase has a color density exceeding the color reproductive threshold density of the copier was developed and used.